Cascade refrigeration systems are well known in the art. In such systems, a first refrigerant of a high stage system is compressed in a high stage compressor, condensed in a high stage condenser, and used, in a high stage evaporator, to condense a second refrigerant compressed by a low stage compressor and then evaporated in a low stage evaporator by absorption of heat from a thermal load, thereby cooling the load. Thus, the high stage system, and notably the high stage heat exchanger, is used as a condenser for the low stage system to condense the second refrigerant by absorption of heat therefrom.
A number of different refrigerants may be deployed in typical cascade refrigeration systems. However, due to environmental concerns, use of many conventional refrigerants containing or releasing CFC (chlorofluorocarbon) base chemicals is becoming less desirable. Instead, use of natural refrigerants or refrigerants having little ozone or global warming impart is increasingly in demand. R-744, or carbon dioxide, is such a refrigerant and is appropriate for use as a second, low stage refrigerant, in a cascade system.
Unfortunately, as with all refrigeration systems, the temperature of the evaporators must be maintained near or slightly below freezing to cool the load to desired temperature, which causes an accumulation of frozen water on the evaporators, and notably the low stage evaporators in cascade systems. Thus, the evaporators must be periodically defrosted. For current cascade systems using R-744 as the low stage refrigerant for the thermal load, the defrosting of the evaporators is effected using air defrost techniques for medium temperature applications, such as display case cooling of non-frozen foodstuffs, and electrical defrost techniques for low temperature applications, such as freezers. Unfortunately, air defrost systems and methods techniques, in which ambient or slightly heated air is blown by fans over the evaporators to melt the ice, are slow and require a great deal of space. Electrical defrost systems and methods, in which heating coils or heaters are deployed in proximity to or on the evaporators, require less time to defrost but use large amounts of electricity.
Accordingly, there is a need for an improved defrost system for a subcritical cascade R-744 refrigeration system.